At present, in the shear speckle interference technique for vehicle tire defect detection, the key is to accurately reflect the real phase of object motion when there are interference factors. Thus, this study proposes a discrete cosine transformation (DCT) unwrapping algorithm based on phase gradient smoothing correction, and analyzes the feasibility of the proposed method through several experiments and numerical simulation calculations. The proposed method replaces abnormal phase gradients caused by noise with calculated weighted moving average phase gradients, bringing phases closer to real values. Finally, the DCT unwrapping method is used to obtain the final solution of phase. A phase recovered using the proposed method is closer to the real value than that recovered using the traditional DCT unwrapping algorithm. Compared to the traditional DCT unwrapping algorithm, the root mean square error accuracy of the proposed method is improved by 10 to 100 times. And after many experiments to record the average running speed, it can be obtained that the time-consumption of the two algorithms only differs by ∼0.1 to 0.3 s.